One method used to manufacture composite or reinforced plastic articles is the pultrusion process. In a fairly simple form, pultrusion is a process in which continuous glass fibers or other reinforcing materials are coated or saturated with a plastic material and then pulled through a die to form an article having a desired cross-sectional profile. The reinforcing material within the article provides strength, durability, and other desirable properties. See, for example, Modern Plastics Encyclopedia, "Pultrusion and Pulforming" (1986).
Pultrusion may be used to prepare articles having nearly any desired cross section. Due to the versatility of the process, pultrusion may be used to prepare a wide variety of structural materials, such as building panels, window lineals, ladder rails, and so on.
While many of the pultruded profiles are hollow because of cost, weight, and processing considerations, for certain applications a composite profile surrounding a filled core is desired. A pultruded profile filled with foam, for example, has superior insulating and structural properties. A pultruded composite with a foam core will have improved resistance to temperature change because the foam will provide increased resistance to the transfer of heat.
Further, a foam core will allow pultruded forms to be joined or have articles secured thereto in the same manner as a more homogeneous material such as wood, due to the good fastener retention capability of structural foams. This fastener retention capability will also provide the ability to fasten parts along the lineal portion of the pultruded profile.
A number of attempts have been made in the prior art to provide a pultruded article with a structural foam core. Staneluis et al., U.S. Pat. No. 4,828,897, describe a method for preparing a reinforced composite having an outer polymeric skin and an inner polymeric foam core bonded together by means of strands of high modulus material. In this process the pressure of the expanding foam material forces the polymeric skin into the mold, thereby forming the proper shape.
McGrath et al., U.S. Pat. No. 5,286,320, teaches the preparation of a pultruded panel by arranging reinforcing fibers on a preformed foam core, applying liquid resin to the reinforcements, and passing the materials through a pultrusion die. Process heat allows the resin to flow and permeate the reinforcements, thereby causing the resin to penetrate both the reinforcements and the core.
In U.S. Pat. No. 5,120,380, an in-line cored pultruded profile is prepared by placing core-forming materials into a pre-die former that is larger than the pultrusion die. As the core-forming material passes through the pre-die former, it progressively conforms to the cross-sectional shape of the die as it approaches the die station.
All of the known methods for forming a foam filled pultruded profile require multiple processing steps, in the pre-forming of a foam core, or added steps to bond the core and the shell, or curing steps.
Accordingly, a need exists for a method involving minimal steps in preparing a foam filled pultruded profile.